The document defines different types of keys used in databases including primary keys, candidate keys, alternate keys, composite keys, and foreign keys. A primary key uniquely identifies each record in a table and cannot be null or duplicate. Candidate keys are attributes that could serve as primary keys. Alternate keys are candidate keys not chosen as primary keys. Composite keys use more than one attribute to uniquely identify records. Foreign keys link records between tables and enforce referential integrity.

1. Types of Keys

2.  Primary Key – This is an attribute or set of attributes that
uniquely indentify a specific instance of a record or an entity.
Every entity in the database must have a primary key whose value
uniquely identifies instances or records of the entity.
Features of the primary key are:
- It must have a non – null value for each instance
- The value must be unique
- The value must not change or become null during the life of each
entity instance

3.  Candidate Key – In some instances, an entity will have more
than one attribute that can serve as a primary key. Any key or
minimum set of keys that could be a primary key is called a
candidate key. Choose the identifier most commonly used by
the user as long as it conforms to the properties of the Primary
Key.
 Alternate Key (Secondary Key) – The Candidate Keys which
are not chosen as the Primary Key are known as alternate
keys.
 Composite Key – Sometimes it requires more than one
attribute to uniquely identify an entity. A primary that is made
up of more than one attribute is known as a composite key.

4.  Foreign Key – A foreign key is an attribute within a child
entity that completes a relationship by identifying the parent
entity. Foreign keys provide a method for maintaining
integrity in the data (called referential integrity) and for
navigating between instances of an entity. Every relationship
in the model must be supported by a foreign key.

5. TRN STUDENT
ID
LAST NAME FIRST
NAME
COURSE ID
100-000-001 SH-1001 ALLEN ROSENNA CHM-101
100-000-002 SH-1002 ALLWOOD PAULA-ANN BIO-101
100-000-003 SH-1003 BRADY BRIANA CHM-101
100-000-004 SH-1004 BURKE TIANA ICT-101